Segregation and influence of boron on the impact toughness of Ti-6 pct Al-2 pct Nb-1 pct Ta-0.8 Pct mo welds and casting
- PDF / 1,021,510 Bytes
- 3 Pages / 594 x 774 pts Page_size
- 75 Downloads / 207 Views
Segregation and Influence of Boron on the Impact Toughness of Ti-6 Pct AI-2 Pct Nb-1 Pct Ta0.8 Pct Mo Welds and Castings H. INOUYE and S. A. DAVID Boron is one of the most effective elements for influencing the properties of several classes of metals and alloys. Trace amounts improve the hardenability of high-strength low-alloy steels, extend the creep rupture life of high temperature alloys, embrittle or ductilize alloys, promote or degrade fabricability, and so forth. In titanium and titanium alloys, boron can be both beneficial and detrimental. In large production ingots of Ti-5 pct A1 (BT-5) modified with 0.01 pct B,* the boron segregated and precipitated as brittle
(Table I) showed no obvious difference that would account for the weld properties; however, spark-source mass spectrometric analyses for trace elements of the weld filler wires and the welds made from them implicated boron as the embrittling impurity in heat B. Evidence that boron was the deleterious impurity was corroborated by the observation that the acicular microstructure in the weld made from heat B was significantly finer than that in welds made from heat A (Figure 1). This agrees with the boron effect on microstructure noted above. To confirm this preliminary finding about the boron effect, we determined the impact toughness of specimens Table I.
Chemical Analyses of Ti-6211 Weld Filler Wire
Element A1 Nb Ta Mo Fe C O N H B Ti
Content (Wt Pct) Heat A Heat B 5.90 5.70 1.96 2.10 0.90 1.10 0.75 0.72 0.04 0.04 0.03 0.03 0.076 0.069 0.012 0.009 0.003 0.006 0.0002 0.0020 Bal Bal
*Alloy compositions in weight percent.
TiBC inclusions, changed the solidification structure, and refined the grain structure. 1 Welds in technical-grade titanium have been inoculated with boron to control the fusion zone grain size. Modification of the weld composition by the addition of 0.01 to 0.03 pct B during welding replaced the acicular microstructure in the unmodified titanium with a disoriented sorbitic type; however, the boron impaired the impact strength. 2 Alloying Ti-4 pct A1-2 pct V with boron refined the alpha plates formed by the /3--~ a transformation, increased the tensile strength and ductility, but reduced the impact toughness. For alloys containing 0.001 to 0.004 pct B, this effect of boron on impact toughness was attributed to a change in the habit plane from {311} in the unmodified alloy to {771} in the modified alloy rather than to the presence of a brittle boride phase because a negligible quantity of precipitates was observed. 3 A detailed analysis of the plate structure in transformed Ti-4.7 pct AI-2 pet V showed that boron refined the average size of the colonies. This refinement of the substructure was attributed to an increase in the number of alpha-phase nuclei. 4 As part of a study to determine the relationship between weld metal properties, microstructure, and segregation of alloying and impurity elements, we illustrate here the deleterious effect of boron on the toughness of Ti-6 pct A1-2 pct Nb1 pct Ta-0.8 pct Mo (Ti-62ll) welds and
Data Loading...
